MAGpurify

This package uses a combination of different features and algorithms to identify contamination in metagenome-assembled genomes (MAGs). Contamination is defined as contigs that originated from a different species relative to the dominant organism present in the MAG.

Each module in the software package was designed to be highly specific. This means that not all contamination (contigs from other species) will be removed, but very few contigs will be incorrectly removed. Feel free to modify the default parameters for more sensitive detection of contamination.

Installation

There is two ways of installing MAGpurify:

• Using conda:

conda install -c conda-forge -c bioconda magpurify

• Using pip:

pip install magpurify

If you install MAGpurify using conda, all dependencies will be installed automatically. However, if you choose to install it through pip, you will need to install some required third-party software:

• BLAST
• Prodigal
• HMMER
• LAST
• Mash
• CoverM

MAGpurify database

Whichever method you choose to install MAGpurify you will need to download a database in order to use some of its modules (see the Dependency on external data section):

• Download the reference database: MAGpurify-db-v1.0.tar.bz2

• Unpack the database:

tar -jxvf MAGpurify-db-v1.0.tar.bz2

• Update your environment:

export MAGPURIFYDB=/path/to/MAGpurify-db-v1.0

If you don't want to put the database into your PATH, you can still use it through the --db parameter of the phylo-markers, clade-markers and known-contam modules.

The MAGpurify database is hosted in Zenodo and can be referenced through a Digital Object Identifier:

A quick overview

The program is broken down into several modules:

$ magpurify

usage: magpurify [-h] [--version]
                 {phylo-markers,clade-markers,conspecific,tetra-freq,gc-content,coverage,known-contam,clean-bin}
                 ...

Identify and remove incorrectly binned contigs from metagenome-assembled
genomes.

positional arguments:
  {phylo-markers,clade-markers,conspecific,tetra-freq,gc-content,coverage,known-contam,clean-bin}
    phylo-markers       find taxonomic discordant contigs using a database of
                        phylogenetic marker genes.
    clade-markers       find taxonomic discordant contigs using a database of
                        clade-specific marker genes.
    conspecific         find contigs that fail to align to closely related
                        genomes.
    tetra-freq          find contigs with outlier tetranucleotide frequency.
    gc-content          find contigs with outlier GC content.
    coverage            find contigs with outlier coverage profile.
    known-contam        find contigs that match a database of known
                        contaminants.
    clean-bin           remove putative contaminant contigs from bin.

optional arguments:
  -h, --help            show this help message and exit
  --version             show program's version number and exit

MAGpurify modules are executed sequentially, usually using the following command structure:

$ magpurify <module> <input mag> <output directory>

After the desired modules have been executed, the flagged contigs are removed:

$ magpurify clean-bin <input mag> <output directory> <output mag>

It's as simple as that!

Dependency on external data

tetra-freq and gc-content don't rely on external data. The phylo-markers, clade-markers and known-contam modules can be run using the standard MAGpurify database. The conspecific module requires that you build your own database. The coverage module requires input BAM files.

An example using the default database

The next few lines will show you how to run the software using a single MAG included with the software.

First, run the individual modules to predict contamination in the example example/test.fna file and store the results in example/output:

$ magpurify phylo-markers example/test.fna example/output
$ magpurify clade-markers example/test.fna example/output
$ magpurify tetra-freq example/test.fna example/output
$ magpurify gc-content example/test.fna example/output
$ magpurify known-contam example/test.fna example/output

The output of each module is stored in the output directory:

$ ls example/output

clade-markers gc-content known-contam phylo-markers tetra-freq

Now remove the contamintion from the bin with clean-bin:

$ magpurify clean-bin example/test.fna example/output example/test_cleaned.fna

• Reading genome bin
  genome length: 704 contigs, 4144.3 Kbp

• Reading flagged contigs
  phylo-markers: 1 contigs, 17.18 Kbp
  clade-markers: 3 contigs, 17.1 Kbp
  conspecific: no output file found
  tetra-freq: 0 contigs, 0.0 Kbp
  gc-content: 0 contigs, 0.0 Kbp
  coverage: no output file found
  known-contam: 0 contigs, 0.0 Kbp

• Removing flagged contigs
  removed: 4 contigs, 34.28 Kbp
  remains: 700 contigs, 4110.03 Kbp
  cleaned bin: example/test_cleaned.fna

In summary, 2 of the 7 modules predicted at least one contaminant and the cleaned bin was written to example/output/cleaned_bin.fna

An example using the conspecific module

To run the conspecific module, you need to build your own reference database using Mash. We've provided some dummy files to illustrate this (but you shouldn't use them with your data!):

$ mash sketch -l example/ref_genomes.list -o example/ref_genomes

This command will create a Mash sketch of the genomes listed in example/ref_genomes.list that are located in example/ref_genomes. The sketch will be written to example/ref_genomes.msh

Now you can run the conspecific module:

$ magpurify conspecific example/test.fna example/output example/ref_genomes.msh

• Finding conspecific genomes in database
  25 genomes within 0.05 mash-dist
  list of genomes: example/output/conspecific/conspecific.list
  mash output: example/output/conspecific/mash.dist

• Performing pairwise alignment of contigs in bin to database genomes
  total alignments: 12125

• Summarizing alignments
  contig features: example/output/conspecific/contig_hits.tsv

• Identifying contigs with no conspecific alignments
  238 flagged contigs, 450.02 Kbp
  flagged contigs: example/output/conspecific/flagged_contigs

So, the conspecific module alone identified 238 putative contaminants! This illustrates that this module can be very sensitive when your MAG is similar to closely related genomes in your reference database… or to other MAGs!

An example using the coverage module

To run the coverage module, you need to input a sorted BAM file containing reads mapped to the MAG (or the original metagenome, as long as the contig name is unchanged). You can also input multiple BAM files and MAGpurify will pick the one with the greatest average contig coverage.

$ magpurify coverage example/test.fna example/output BAM/sample_1.bam BAM/sample_2.bam BAM/sample_3.bam

• Computing contig coverage

• Identifying outlier contigs

• Sample being used for outlier detection: sample_2
  2 flagged contigs: example/output/coverage/flagged_contigs

Details on the individual modules

phylo-markers

This module works by taxonomically annotating your contigs based on a database of phylogenetic marker genes from the PhyEco database and identifying taxonomically discordant contigs.

clade-markers

This module works in a very similar way to phylo-markers, but instead uses clade-specific markers from the MetaPhlAn 2 database for taxonomic annotation.

conspecific

The logic behind this module is that strains of the same species should have similarity along most of the genome. Therefore, this module works by first finding strains of the same species, and then performing pairwise alignment of contigs. Contaminants are identified which do not align at all between genomes.

tetra-freq

This module works by identifying contigs with outlier nucleotide composition based on tetranucleotide frequencies (TNF). In order to reduce TNF down to a single dimension, principal component analysis (PCA) is performed and the first principal component is used.

gc-content

This module works by identifying contigs with outlier nucleotide composition based on GC content.

coverage

This module works by identifying contigs with outlier coverage based on read mapping information.

known-contam

This module works by identifying contigs that match a database of known contaminants. So far, the human genome and phiX genome are the only ones in the database.

Citation

If this code is useful, please cite:

Nayfach, Stephen, et al. "New insights from uncultivated genomes of the global human gut microbiome." Nature 568.7753 (2019): 505-510.